For example, in a trawling method, a scanning sonar is effectively used to search for a fish school to be found, tracked and captured. With a search for a fish school, although detectability varies depending on the size of the fish school, a school of sardine or herring, for example, can be searched from several hundred to several thousand meters away. When a fish school is found, fish tracking is performed by turning or controlling a ship so that the fish school is positioned in the bow direction of the ship, and after catching up with the fish school and running just upon the fish school, the fish school is captured by being driven into a backward trawl net of the ship as the ship runs. At that time, the point is how the net mouth of the trawl net is suitably guided to the fish school.
The position and depth of the net mouth can be controlled by ship handling and towing speed. However, for that purpose, the position and speed of the net mouth have to be grasped accurately. Accordingly, in the trawling fishing method, it is not too much to say that the fishing efficiency depends on the performance of an apparatus for monitoring the position and depth of the net mouth.
Hitherto, the position and depth of the net mouth can be monitored by a method in which a transponder is attached to the net mouth, a response signal of the transponder is received by two or three wave receivers attached to the bottom of the ship, and a phase difference of those received signals is obtained to know the direction of the transponder, that is, the direction of the net mouth. Besides, JP-B-1-53751 (patent document 1) or JP-UM-B-5-2874 (patent document 2) disclose a system in which a scanning sonar is used, plural transponders are provided at the center of a net, and a fish school signal (echo) and a net position signal using the same frequency are displayed in a superposition state.
In the former method using the phase difference measurement, signals reflected by the sea surface or the sea bottom and through multi-pass routes are mixed to the direct wave from the transponder, and there has been a defect that a wrong direction is erroneously detected or position detection is difficult to perform stably. Besides, in the system disclosed in patent document 1 or 2, there have been following problems to be solved.
First, with respect to the detection of the transponder, the system disclosed in patent document 2 requires another display device dedicated to the transponder for displaying the echo signal. Besides, in order to detect the frequency, specific units such as a pressure sensor and a VF conversion circuit are required. Further, since the transponder is detected by a vertically wide beam, there has also been a problem that a high SN ratio can not be obtained. In the system disclosed in patent document 1, there has been a problem that it is impossible to distinguish between the echo signal and the response signal of the transponder since their frequencies are the same. Besides, with respect to both systems disclosed in patent documents 1 and 2, when the receiving frequency band is made narrow in order to raise the detection probability of the response signal of the transponder, the influence of the Doppler shift comes to be liable to exert. That is, the SN ratio and the Doppler shift resistance have the relation of trade-off.
With respect to the position display of the transponder, the system disclosed in patent document 1 has a problem that the depth information of the transponder is difficult to grasp.
Besides, with respect to the activation of the transponder, in the system disclosed in patent document 1, activation of the transponder and detection cannot be optimized independently because they are processed in common. For example, it has been necessary that signals for transponder activation and for detection have to be transmitted and received at the same tilt angle. Besides, there has been a problem that the sound pressure of the signal to the transponder lowers as the distance to the transponder elongates, and, as a result, the activation of the transponder becomes difficult.
An object of the invention is therefore to provide an ultrasonic transmitting and receiving system in which the foregoing problems are solved, a specified detection area can be detected, and a position of a transponder put in water can be certainly monitored.